Magnesium-titanium catalysts for olefin polymerization are in wide commercial use. In general, these catalysts comprise a magnesium halide component (typically, magnesium dichloride) and a titanium component that is deposited on the magnesium dichloride. The catalyst is generally activated with a hydrocarbyl aluminum activator. The catalyst system is often used in supported form (with silica, alumina or silica-alumina supports being well known) but may also be used in the absence of such a support (in which case, the magnesium dichloride may be regarded as a “support”).
The use of very finely divided magnesium halide particles is generally preferred. One well-known method to produce finely divided magnesium dichloride is to react a hydrocarbon soluble organomagnesium compound (such as diethyl magnesium) with a source of active chlorine. The active chlorine source is typically selected from the group consisting of 1) hydrochloric acid, HCl, 2) non-metallic halides such as isopropyl chloride, secondary butyl chloride or tertiary butyl chloride and 3) active metal chlorides, (especially aluminum organochlorides or aluminum trichloride).
The amount of active chlorine is typically specified to be sufficient to react with substantially all of the organic ligands on the organomagnesium compound, as described in U.S. Pat. No. 4,612,300.
A titanium species is generally then added to the magnesium chloride. The resulting magnesium-titanium complex is often referred to as a “procatalyst” because it requires a co-catalyst or an activator to produce a highly reactive polymerization catalyst system.
The procatalyst may be first synthesized then added to the polymerization reactor at a later time, as disclosed in U.S. Pat. No. 4,612,300. Alternately, the procatalyst may be prepared by an ‘in-line mixing technique’ (adjacent to a polymerization reactor) and added directly to the reactor, as disclosed in U.S. Pat. No. 6,723,677.
A hydrocarbyl aluminum species (especially triethyl aluminum) is commonly used as the co-catalyst or activator. It is generally preferred to add at least a portion of the co-catalyst/activator directly to the polymerization reactor.
Many of the original Ziegler-Natta catalysts were not sufficiently active to permit the catalyst residues to be left in the polymer without causing quality problems (such as polymer color and a propensity to degrade/oxidize the polymer in an undesirably short time period). Accordingly, there is a need for “high activity leave-in” catalysts, which are characterized by having less problematic catalyst residues that may be left in the finished polymer.
It is especially difficult to prepare a “high activity leave-in catalyst” for the solution polymerization of thermoplastic polyolefins because the comparatively high polymerization temperatures required for such polymerizations are known to cause the deactivation of magnesium-titanium catalysts.
We have now discovered a highly active magnesium-titanium catalyst that is especially suitable for the solution polymerization of thermoplastic polyolefins.